[dev.power64] runtime: fix gcinfo_test on power64x

The GC info masks for slices and strings were changed in
commit caab29a25f68, but the reference masks used by
gcinfo_test for power64x hadn't caught up.  Now they're
identical to amd64, so this CL fixes this test by combining
the reference masks for these platforms.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/162620044